1. Field of the Invention
The present invention relates to a display device integrated with a touch panel, and more particularly, to a liquid crystal display device integrated with a touch panel capable of using various input devices, including conductive and nonconductive types, and its fabrication method.
2. Discussion of the Related Art
For personal information devices, such as personal computers and mobile transmission devices, a variety of input devices, such as a keyboard, a mouse and a digitizer, have been generally used for text and graphic processes. As personal information devices are in demand in various fields, the input devices of the keyboard and the mouse have a limit in coping with the demand as an interface. Thus, it is beneficial to develop an input device that is easier to carry and simpler than those conventional input devices. That is, a user can input information such as letters on the input device with a bare hand while carrying the input device. In recent years, modern input devices have been developed not only to satisfy general input functions, but to have new functions and to provide them with great reliability and endurance using a high technology.
Touch panels are known as an input device that are simple, easy to carry, reliable and capable of inputting letters. The function and detection method of such touch panels are described in detail hereafter. Capable of sensing when a user touches a display surface, touch panels may be classified into resistive type, capacitive type, and electromagnetic (EM) type. For the resistive type touch panels, a metal electrode is formed either on an upper substrate or on a lower substrate, and the location of a touched point is detected by reading a voltage gradient created by a resistance at the touched point in an applied D.C. voltage. The capacitive type touch panels detect the location of a touched point based on a voltage change created when an upper substrate having a conductive layer of an equipotental plane is in contact with a lower substrate. Also, the EM type touch panels detect the location of a touched point by reading an induced LC value when a conductive layer is touched with an electronic stylus pen. Since each type has different characteristics of signal amplification, resolution, and difficulty of design and fabrication, a type is chosen for specific applications, such as optical, electrical, mechanical, resistance to ambient atmosphere and input characteristics as well as endurance and economical efficiency.
Hereinafter, a related art touch panel will be described with reference to the accompanying drawings.
FIG. 1 is a schematic view illustrating a general capacitive type touch panel and its operation principle. As shown in FIG. 1, the general capacitive type touch panel includes metal electrodes 2a, 2b, 2c and 2d at each corner of a curved or plane glass substrate (10 of FIG. 2) coated with a transparent conductive layer 1, for forming an equipotential surface. That is, a voltage is applied to the transparent conductive layer 1 through the metal electrodes 2a, 2b, 2c and 2d for forming an equipotential surface thereon. When a surface of a touch panel is touched, a voltage drop is generated. In the general capacitive type touch panel, the amount of the voltage drop is detected with a controller, thereby detecting the location of the touched point. In this case, the input device of the capacitive type touch panel may be a bare finger or a conductive stylus pen. That is, the input device induces a voltage drop at a touched point when an input surface is touched.
More specifically, the related art capacitive type touch panel will be described with reference to the following plan and cross-sectional views. FIG. 2 is a plan view illustrating a capacitive type touch panel, and FIG. 3 is a cross-sectional view taken along line I-I′ of FIG. 2.
As show in FIGS. 2 and 3, in the capacitive touch panel, the glass substrate is formed as a supporting substrate. Then, the transparent conductive layer is formed on the glass substrate, and the metal electrodes are formed at four corners of the transparent conductive layer for applying a voltage to the transparent conductive layer. Beneficially, the transparent conductive layer 1 is formed of a transparent and conductive material such as Indium-Tin-Oxide (ITO) or Antimony-Tin-Oxide (ATO), and the glass substrate 10 is formed of Soda-Lime Glass.
Then, the metal electrodes 2a, 2b, 2c and 2d are formed at the four corners of the transparent conductive layer 1 by printing a conductive metal having a low resistance, such as Ag. Also, a resistance network is formed around the metal electrodes 2a, 2b, 2c and 2d. The resistance network is formed in a linear pattern for uniformly transmitting control signals to an entire surface of the transparent conductive layer 1. Although not shown, a passivation layer is coated on the transparent conductive layer 1 including the metal electrodes 2a, 2b, 2c and 2d. The passivation layer may be formed of a liquid glass material. However, a heat treatment is performed to the liquid glass material to turn it into a hard coating using a hardening/densifying process.
FIG. 4 is a block diagram illustrating an operation principle of a general capacitive type touch panel. As shown in FIG. 4, a voltage is applied to the metal electrodes 2a, 2b, 2c and 2d at the four corners of the transparent conductive layer 1 in the general capacitive type touch panel. A high frequency voltage is beneficially transmitted to the entire surface of the touch panel. At this time, if the transparent conductive layer 1 is touched with a finger or a conductive stylus pen, a current change is detected by each of the current sensors 20a, 20b, 20c and 20d. The current sensors 20a, 20b, 20c and 20d as well as an analog-digital converter 30 correspond to the controller (reference numeral 3 of FIG. 1). The controller 3 provides a square wave having a frequency on the order of several kilohertz Hz to the current sensors 20a, 20b, 20c and 20d through an internal oscillator OSC, and the current sensors 20a, 20b, 20c and 20d continuously charge and discharge. Then, the amount of charge is integrated in a digital mode during an integral period (usually 8 msec). In this way, it is possible to determine whether the input surface is touched. If the input surface is touched, the coordinates of the touched point are measured, and then is outputted to a system (not shown) as information. In the general capacitive type touch panel, the input means are restricted to bare fingers or conductive stylus pens. Accordingly, when a gloved hand is used or when a conductive stylus pen is unavailable, it is hard for the general capacitive type touch panel to detect the location of a touched point correctly.
Meanwhile, for a general resistive type touch panel, when a surface of an upper substrate having an upper electrode thereon is touched with input means, such as stylus pens or fingers, the upper electrode of the upper substrate is electrically connected to a lower electrode of a lower substrate. Thus, a voltage change is read according to a resistance value at the touched point, and then the coordinates of the touched point can be determined according to the potential change in the controller. Therefore, it is possible for the resistive type touch panel to detect the location of a touched point with a gloved hand or a non-conductive stylus pen, irrespective of whether the input device is conductive. However, unlike the capacitive type touch panel, since the resistive type touch panel detects the location of a touched point according to the resistance of the transparent conductive layer, it suffers from inaccuracy and low resolution, compared with the capacitive type touch panel.